Two heavy balls are simultaneously shot from two spring toy-guns arranged on a horizontal plane at a distance s = 10 m from each other. The first ball has the initial vertical velocity `v_(1) = 10 m//s`, while the second is shot at an angle `alpha` to the horizontal at a velocity `v_(2) = 20 m//s`. Each ball experiences the action of the force of gravity and the air drag `F = mu v, mu, = 0.1 g//s`. Determine the angle cc at which the balls collide in air.